Entrapment of Water Droplets in Wax Deposits ... - ACS Publications

Subscriber access provided by University of Otago Library

Article

Entrapment of Water Droplets in Wax Deposits from Waterin-Oil Dispersion and Its Impact on Deposit Build-up Sheng Zheng, Thanaphoom Khrutphisit, and H. Scott Fogler Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02450 • Publication Date (Web): 22 Nov 2016 Downloaded from http://pubs.acs.org on November 29, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Energy & Fuels is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 59

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

11-18-2016

Entrapment of Water Droplets in Wax Deposits from Water-in-Oil Dispersion and Its Impact on Deposit Build-up Sheng Zheng†, Thanaphoom Khrutphisit†,§ and H. Scott Fogler†,* †

Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109

§ The Petroleum and Petrochemical College, Chulalongkorn University, Bangkok 10330, Thailand * Contact:[email protected]

ABSTRACT Wax deposition from water-in-oil emulsion/dispersion can entrap water droplets in the resulting paraffin deposits. This entrapment of water droplets can lead to a significant decrease in the yield stress of the deposits, which can cause the deposit to slough off during wax deposition. This investigation presents the most comprehensive characterization of the entrapment of water droplets by deposit formation with cold finger and flow loop wax deposition experiments. NMR techniques were implemented to characterize the droplet size of the bulk and deposit in order to reveal the relationship between the droplet size of the bulk and the water content of the deposit. Furthermore, it is demonstrated that incorporation of droplets in the deposit lowers the yield stress of deposit and causes the deposit to slough-off

1 ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

11-18-2016 during pipe flow. Knowledge of such effects of the water phase on wax deposition is beyond conventional analysis based on heat and mass transfers and is expected to provide additional insights to the design of water-oil two phase transport pipelines.


Page 2 of 59

Page 3 of 59

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

11-18-2016 1. INTRODUCTION Water is commonly co-produced with oil. As a result, water and oil usually co-exist in the production stream from petroleum reservoirs1. The production stream is mainly composed of hydrocarbons and with low water content at early stages of well production because hydrocarbons usually lie above the aquifer and flow out of the well before water. The water content of the production stream likely continues to increase over time2. Moreover, at the late life-time of the well, water-flooding is implemented, resulting in a significant amount of water in the production stream3-5. It is interesting to notice that wells producing a water content higher than 90% can still be economically valuable6. During pipeline transportations of the crude, co-existence of water and oil in the production stream can generate complex multiphase flow patterns, including water-in-oil dispersed flow7,8, oil-in-water dispersed flow9,10, annular flow11, stratified flow12, etc. Additionally, crude oil usually contains natural surfactants, e.g., resins and asphaltenes13-16. These natural surfactants promote the formation of water-in-oil dispersion, making it a common multiphase flow regime in oil production. In the water-in-oil dispersed flow pattern, the oil phase is in contact with the pipe wall. As a result, wax deposition can occur when the temperature of the inner pipe wall is below the wax appearance temperature7,8,17. Wax deposition poses severe risks to off-shore pipelines transporting oil. The presence of dispersed water droplets can affect the wax deposition characteristics. Bruno et al, Couto et al., Panacharoensawad and Sarica and Zhang et al17-20. performed wax deposition experiments with cold finger and/or flow loop apparatus using crude oils. It was observed that wax deposition rate decreases with increasing water content in the bulk. Similar trend in the effect of water on wax deposition rate was also observed by Kasumu and Mehrotra21. Wang et al. and Quan et al. observed non-monotonic variation in


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

11-18-2016 the deposition rate with increasing water content22,23. The investigators explained the nonmonotonic variation in the wax deposition rate based on a combination of diffusion and gelation mechanism. Wang et al. further advanced the understanding of the gelling deposition mechanism on wax deposition and proposed an algebraic model to calculate deposition rate based on this mechanism24. Different from wax deposition from single phase oil flow, it was observed that the deposits formed from water-in-oil dispersed flow could contain water droplets7,8,17. It should be noted that entrapment of water droplets in the deposit significantly alters the rheological properties of the deposit, including yield stress25, storage/loss modulus and thixotropy26. For example, it was observed that incorporation of water droplets in the deposit microstructure lowered the deposit yield stress25. Among the impact of water droplets on the rheological properties of the deposit, the change in the deposit yield stress has the most relevant influence on wax deposition characteristics as a lowered yield stress makes the deposit prone to slough-off by the shear stress imposed by the flow27. Without the consideration for deposit slough-off, existing wax deposition models will over-predict the deposit growth rate and lead to unnecessarily conservative scheduling of the costly pigging operation. The yield stress of the deposit varies with the water content of the deposit25,26,28. As a result, knowledge of the water content of the deposit is essential for the estimation of deposit yield stress and assessment for the probability of slough-off of wax deposit. Unfortunately, the characterization of water content of deposit has not received due attentions in previous experimental campaigns to study wax deposition from water-in-oil two phase flow7,8,17,21. Figure 1 summarizes previous reports of the water volume fraction of the deposit generated with water-oil mixtures17,21.


Page 4 of 59

Page 5 of 59

11-18-2016 100

Water Content of Deposit (wt.%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Bruno et al. flow loop experiments Kasumu et al. flow loop experiments 45 degree line

80 60 40 20 0 0

20

40 60 80 Water Content of Dispersion(wt.%)

100

Figure 1: Summary of Reported Characterizations of the Water Content in the Deposit in Comparison with the Water Content of the Dispersion17, 21 As can be seen from Figure 1, no obvious correlation between the water contents of the bulk liquid and the deposit has been observed in previous experimental investigations. Interestingly, in some experiments17, 21, the deposits contain virtually no water while the water content of the bulk can be as high as 40 wt.%. Because of the existing inconsistencies reported in the water content of deposits formed from water-in-oil dispersions, a fundamental understanding of the entrapment of water droplets during the deposition process is still lacking. A comprehensive characterization of the water content and the droplet size distribution of the deposit is reported in this study. Based on this characterization, the inconsistencies in the reported water content of deposits17, 21 are resolved. In addition, the slough-off of wax deposits due to the entrapment of water droplets is demonstrated with a flow loop apparatus.


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 59

11-18-2016 2. EXPERIMENTAL 2.1 Model Oils Two waxy model oils, Oils 1 and 2 were prepared and used as the oil phases for the experiments. The compositions, wax contents and wax appearance temperatures (WAT) of these two model oils were summarized in Table 1. The WAT’s of the model oils were measured by cooling the model oils in a rheometer and monitoring the viscosity. The WAT is defined as the temperature below which the viscosity-temperature trajectory deviates from an Arrhenius temperature dependence. Table 1: Summary of the Formulation of the Two Waxy Model Oils Used in this Study Model Oil 1 2

Solvent Crystal Plus 70T Mineral Oil

Wax Wax A: 327204 Aldrich, melting point 53°C-57°C Wax B: 411663 Aldrich, melting point ≥ 65°C

Wax Content (wt.%)

WAT (°C)

3

20

5

38

The solvent used in both model oils is the Crystal Plus 70 T mineral oil purchased from STE Oil Company. The basic physical properties of this mineral oil are shown in Table 229. Table 2: Basic Physical Properties of the Solvent in the Model Oils

Density at 20 °C (g/mL) 0.857 Viscosity at 40°C (c.P.) 10.2 Averaged Molecular Weight (g/mol) 311 Figure 2 shows the carbon number distributions of the waxes in the two model oils. The carbon number distributions were measured by an Agilent Technogolies 6890 N high temperature gas chromatograph (HTGC).


Page 7 of 59

11-18-2016 0.2 Wax A

Weight Fraction

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Wax B

0.16 0.12 0.08 0.04 0 15

20

25

30

35

40

45

Carbon Number Figure 2: Carbon Number Distributions of the Waxes Used in the Preparation of Model Oils The first wax, wax A, with a lighter carbon number distribution between the two waxes is used to generate a model oil with a WAT (20 °C) lower than room temperature (25 °C). With this model oil, the water droplets in the deposit can be inspected easily at room temperature without the interference from precipitated wax particles as wax molecules remain dissolved at room temperature. The second wax, wax B, with the heavier carbon number distribution is used to generate a model oil with a WAT (38°C) higher than room temperature (25 °C) to induce stabilization of water-in-oil dispersion with suspended wax particles. Such stabilization by suspended wax particles is critical to allow for the characterization of droplet size distribution of dispersions with low surfactant dosages.

2.2 Preparation of Model Emulsion The mineral oil used in this study is free of natural surfactants such as resins and asphaltenes contained by crude oils. Consequently, synthesized surfactants were added to the model oil in order to facilitate emulsification. The surfactant used to generate the emulsion/dispersion


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

11-18-2016 is a 9:1 (based on mass) mixture of sorbitan monooleate (SPAN 80) and sodium di-2ehtylhexylsulfosuccinate (AOT). According to previous experimental studies30, the water-inoil mixtures generated with a surfactant concentration higher than 0.5wt.% (based on the total mass of oil and water) present long-term stability against phase separation. Therefore, these mixtures will be called “emulsion” from this point onward. Preliminary experiments in this investigation show that the water-in-oil mixtures with a surfactant concentration in the range between 0.1 wt.% and 0.5 wt.% undergo rapid phase separation when kept stationary while can be homogenized with stirring in the cold finger apparatus or with pumping in the flow loop apparatus. Because of the significantly different bulk stability characteristics compared to the stable emulsions, these mixtures with low surfactant dosages will be called “dispersion” from this point onward. In order to prepare emulsion/dispersion for wax deposition experiments in the cold finger apparatus/flow loop, the oil phase was prepared by first dissolving various amount of the surfactant mixture (0.1wt.% to 1.0wt.% based on the total mass of oil and water) in the model oils. In the cold finger apparatus, emulsification/dispersion of water was achieved by adding deionized (DI) water with a volume fraction varying from 10% to 70% to the oil phase at a temperature above the WAT while under intensive mixing with a mechanical stirrer. In the flow loop apparatus, the emulsion/dispersion was prepared by addition of DI water to the reservoir containing oil followed by circulation of the water-oil mixture in the flow loop apparatus until the pressure drop reading across the test section reaches a steady level.

2.3 Cold Finger Wax Deposition Experiments Figure 3 shows a schematic drawing of the cold finger apparatus.


Page 8 of 59

Page 9 of 59

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

11-18-2016

Figure 3: A schematic Drawing of the Cold Finger Apparatus In order to prepare for a wax deposition experiment with the cold finger apparatus, the temperatures of the reservoir as well as the cold finger were first adjusted to the desired set points T1 and T2, (T1 > T2 and T2 < WAT) by setting the temperatures of two thermal baths. Once the desired set points were reached, the cold finger was inserted into the water-oil mixture in the reservoir containing waxy emulsion/dispersion to start wax deposition.

2.4 Flow Loop Experiments Flow loop wax deposition experiments were performed under ambient pressure with the newly built Michigan Flow Loop. Figure 4 shows a schematic drawing of the flow loop apparatus.


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

11-18-2016

Figure 4: Schematic Drawing of the Michigan Flow Loop. Labeled Sections Refer to: 1) Conditioning System, 2) Pumping System, 3) Testing System and 4) Data Acquisition System The flow loop apparatus consists of four key parts: the conditioning system, the pumping system, the testing system and the data acquisition system. The flow loop test section is a double pipe heat exchanger with an inner diameter of 0.4 inch and a length of 4 feet. A hot waxy oil/water-in-oil mixture and a cold coolant stream flow in a counter-current configuration to generate wax deposition. Thermal couples were installed at the inlet and outlet of the test section to monitor the temperatures of the waxy oil/water-in-oil mixture and the coolant at these locations. Pressure transducers were installed at the inlet and the outlet of the test section in order to monitor the build-up of pressure drop due to deposit formation. The test section can be detached from the flow loop apparatus for sampling of wax deposit at the end of wax deposition experiments. In order to perform wax deposition experiments, wax and surfactants were dissolved in the mineral oil in the reservoir. Water-in-oil dispersion is prepared by addition of water into the reservoir followed by circulation of the water-in-oil mixture in the flow loop apparatus at the desired flow rate. It should be noted that only “model oil 2” is used for the flow loop wax deposition experiments as the wax 10 ACS Paragon Plus Environment

Page 10 of 59

Page 11 of 59

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

11-18-2016 deposition rate achieved with “model oil 1” is unreasonably low due to its low wax content and wax appearance temperature.

2.5 Characterization for Water Content and Droplet Size Distribution In order to measure the water content of the deposit, the deposit sample was first dissolved in a 1:1 (based on volume) mixture of toluene and acetone. The water content was then measured with a Mettler Toledo V20 Volumetric Karl-Fischer Titrator. Two techniques were used to characterize the droplet size distribution. An optical microscope was used to characterize the droplet size distribution of stable emulsions generated with a surfactant dosage higher than 0.5wt.% as well as the deposits generated from stable emulsions. In order to carry out this characterization, a liquid film of the sample was prepared by placing a drop on a glass slide followed by compressing with a cover slide. The microstructures of the emulsion and deposit were inspected by a Nikon Eclipse E600 microscope. A 50×objective lens and a 10× eyepiece were attached to the microscope. A Sony ACV-D7 CCD camera was used to take photos. With model oil 1, the wax molecules dissolve in the oil phase at room temperature. Without the interference from wax particles, the droplet size distribution can be measured based on the micrographs using imageprocessing software, ImageJ. For dispersions generated with a lower surfactant concentration (0.1wt.% to 0.2wt.%), phase separation occurs rapidly in absence of wax particles. Therefore, the droplet size distribution can only be characterized in presence of wax particles, which retard phase separation. As a result, such characterization is only possible with the deposits generated with model oil 2 whose WAT is significantly higher than the room temperature. Unfortunately, suspended wax particles then cause the micrographs of the dispersion/deposit to become hazy. As a result, a non-optical method is 11 ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

11-18-2016 necessary for the characterization of droplet size distribution for the water-in-oil dispersions and the deposit generated from dispersions. The DOSY-NMR technique31-33 is used for this characterization. DOSY-NMR measures the 1H signal attenuation with increasing pulse field gradient, which is due to the Brownian motion of H2O molecules. This signal attenuation becomes more profound if the Brownian motion of the molecules generates a large mean square displacement. As a result, Brownian motions of water molecules enclosed by a coarse droplet will lead to a larger signal attenuation compared to the Brownian motions of molecules enclosed by a fine droplet as the boundaries of droplets limit the mean square displacement of the Brownian motions. Based on this principle, the measured signal attenuation can be used to back-calculate the droplet size distribution. The DOSY-NMR experiments in this investigation were performed on a 400 MHz Varian MR400 NMR spectrometer at the University of Michigan NMR facility.

2.6 Characterization of the Yield Stresses of Waxy Gels and Waxy Emulsion Gels The yield stresses of the waxy gels/waxy emulsion gels were measured with an AR2000 controlled stress rheometer. The yield stress tests were performed with model oil 2 and the corresponding stable emulsions (with a surfactant concentration of 1wt.%) with water contents of 10, 30, 50 and 70 vol.%. These emulsions were generated with a Scilogex D160 homogenizer. Before testing for the yield stress, a gel is first formed in the geometry gap of the rheometer by cooling of the waxy model oil/emulsion from ~10 °C above its WAT to ~30°C below its WAT. The evolution of the storage and loss moduli were probed with an oscillatory stress of 1 Pa and an oscillatory frequency of 0.1 Hz during cooling of the sample. The gelation temperature is defined as the cross-over point between the storage modulus – temperature and the loss modulus – temperature trajectories. The shear stress imposed on the 12 ACS Paragon Plus Environment

Page 12 of 59

Page 13 of 59

11-18-2016 gel was then increased at a ramp rate of 20 Pa/min and the viscosity was monitored during this stress ramp test. The shear stress at which the viscosity decreases drastically is defined as the yield stress of the gel, as shown in Figure 5.

Viscosity (Pa•s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

yield stress = 100 Pa

Figure 5: Determination of the Yield Stress of a Waxy Gel/Waxy Emulsion Gel Based on the Viscosity-Stress Trajectory For the yield stress tests, a serrated bottom plate was mounted on top of the smooth bottom plate and a serrated 40 mm plate was used as the top geometry. Use of such serrated surfaces prevents slippage between the sample and the bottom plate/top geometry and improves the reproducibility of the tests. As can be seen from Figure 5, excellent reproducibility in the yield stress measurements can be achieved with the serrated bottom plate and top geometry.

3. COLD FINGER EXPERIMENTS Cold finger wax deposition experiments were first performed to investigate the entrapment of water droplets in the deposit. Table 3 summarizes the experimental runs and the variables investigated.


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

11-18-2016 Table 3: Cold Finger Experimental Matrix Group 1: Investigate of the effect of bulk water content and the time-evolution of the water content of deposit Experiments in this group were performed under ambient pressure, an oil reservoir temperature of 15 °C and a finger temperature of 5°C Experimental Run No. Water Content in the Bulk (vol.%) Surfactant Loading (wt.%) Duration (hr) 1-4 1/12 5-8 1/4 9-12 1/2 10,30,50,70 1 13-16 1 17-20 2 Group 2: Investigate the effect of bulk droplet size on the deposit water content Experiments in this group were performed under ambient pressure, an oil reservoir temperature of 50 °C and a finger temperature of 5°C Experimental Run No. Water Content in the Bulk (vol.%) Surfactant Loading (wt.%) Duration (hr) 21 0.1 22 0.15 23 0.2 50 1/2 24 0.5 25 1

3.1 Water Content and Droplet Size Distribution of Wax Deposits from Stable Water-in-Oil Emulsion (with Model Oil 1) in a Cold Finger Cold finger wax deposition experiments were first performed with a stable water-in-oil emulsion using model oil 1 with varying Water Contents (WC) from 10 (WC = 10) to 70 vol.% (WC = 70). For WC = 10 and WC = 30 emulsions, the deposition experiments were performed under stirring while the experiments with WC 50 and WC 70 were performed under stationary conditions as stirring is not possible owing to the high viscosity of these emulsions with high water contents. Figure 6 shows the comparison between the water contents of the water-in-oil emulsion in the cold finger and the deposit.


Page 14 of 59

Page 15 of 59

11-18-2016 100

Water Content of the Deposit (wt.%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

80 60 40

Phase inversion point

20 0 0

20 40 60 80 100 Water Content of Emulsion(wt.%)

Figure 6: Comparison between the Water Content in the Deposit with that in the Emulsion* As can be seen from Figure 6, the water content of the deposit equals that of the bulk for all experiments performed with stable water-in-oil emulsions before phase inversion occurs. In order to determine the phase inversion point, the viscosities of the water-in-oil emulsions with various water contents were measured. The phase inversion point was determined as the water content where a maximal viscosity was observed. More details of the determination of the phase inversion point can be found in the supporting information. Figure 6 suggests that the water content of the deposit can be as high as that of the emulsion, i.e., the water content of the emulsion is the upper bound of the water content of the deposit. This upper bound is reported here for the first time. None of the previous experimental investigations could establish this upper bound17, 21. All previous experimental investigations reported deposits with lower water content than that of the bulk17, 21. Moreover, no apparent correlation was observed between the water contents of the deposit and bulk. Consequently, in order to support the experimental findings in this

*

Different data points at the same bulk water content represent water contents of the deposits obtained with various deposition durations of 5 minutes, 15 minutes, 30 minutes, 60 minutes and 120 minutes


Energy & Fuels

11-18-2016 study, the droplets in the deposit were inspected using an optical microscope. Figure 7 shows the micrograph of the deposit and the emulsion.

Figure 7: Comparison between the Microstructures of Deposit and Emulsion As can be seen from Figure 7, water droplets remain dispersed in both the emulsion and the deposit. Figure 8 shows the comparison between the droplet size distribution of the deposit and that of the emulsion extracted from image analysis based on micrographs.

Fraction of Droplets

0.5

0.5 Water Content (WC) = 10 vol.%

0.4

WC 30

0.4

deposit emulsion

0.3 0.2

0.3 0.2

0.1

0.1

0.0

0.0

0

2 4 6 8 10 Droplet Diameter (µm)

0

0.5


0.6 WC 50



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 59

0.4 0.3 0.2 0.1 0.0

WC 70

0.5 0.4 0.3 0.2 0.1 0.0

0


0


Figure 8: Comparison between the Droplet Size Distributions of the Deposit and the Emulsion


Page 17 of 59

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

11-18-2016 As can be seen from Figure 8, the droplet size distributions in the deposit and in the emulsion are virtually the same. The results shown in Figure 8 are the first reported comparisons of droplet size distribution in the deposit and in the emulsion. Identical water contents and droplet size distributions of the deposit and emulsion suggest that water droplets are entrapped “in-situ” in the void space enclosed by the interlocking network of wax particles during deposit formation.

3.2 Time Evolutions of the Water Content and Droplet Size Distribution from Stable Water-in-Oil Emulsion in a Cold Finger Wax deposition is a long term process that can occur over a time period on the order of weeks. During the process of wax deposition, it has been shown that the deposit thickness as well as the deposit wax content increase over time34-37. However, the time evolution of the water content in wax deposit has not been well understood. Consequently, cold finger wax deposition experiments were performed with various durations in order to understand the time evolution of the water content and droplet size distribution. Figure 9 shows an example of the increases in the deposit weight and wax content over a 2-hour period, confirming deposit growth and aging.


Energy & Fuels

2

12

1.6

10 8

1.2

6 0.8

4

0.4

2

0

0 0

40

80

Deposit Wax Content* (wt.%)

Weight of Deposit (g)

11-18-2016

120

Time (minute)

Figure 9: Time Evolution of the Deposit Weight and Wax Content with a Water Content of 10 vol.% Figure 10 (a) shows the evolution of the water content of deposit over a deposition period of 2 hours. The theoretical water content was calculated based on the amount of oil and water added to the reservoir at the beginning of the wax deposition. Figure 10 (b) shows the evolution of deposit droplet size distribution over the same deposition period. 0.5

12 8

theoretical value

4 0


16


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 59

Emulsion 5 minutes Deposit

0.4

15 minutes Deposit

0.3

30 minutes Deposit 60 minutes Deposit

0.2

120 minutes Deposit

0.1 0.0

0

20

40 60 80 100 120 Time (minute)

0

(a)


(b)

Figure 10: Time Evolutions of (a) Water Content and (b) Droplet Size Distribution of the Deposit Generated from Emulsion with Water Content of 10 vol.% As can be seen from Figure 10, the water content of the deposit as well as the droplet size distribution remain constant during the process of deposit growth. Constant water content

*

Deposit wax content is defined as mwax/(mwax+mdeposit) and the mass of water is not included. 18 ACS Paragon Plus Environment

Page 19 of 59

11-18-2016 during wax deposition was also observed from tests performed with higher water contents (30 vol.%, 50 vol.% and 70 vol.%), shown in Figure 11. 80


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

60

Lines represent water content values calculated based on the masses of oil and water added.

40

20

0 0

20

40 60 80 Time (minute)

100

120

Figure 11: Time Evolution of Water Content of the Deposit Generated from Emulsion with Water Contents of 10, 30, 50 and 70 vol.% The evolution of the water content and droplet size distribution of the deposit with time, shown in Figure 10 and Figure 11 respectively, reveal the mechanism of droplets entrapment by the deposit. That is, the water content and droplet size distribution of deposit do not vary with time, suggesting the water droplets are entrapped “in-situ” in the deposit solid matrix.

3.3 Effect of Droplet Size on the Entrapment of Water during Wax Deposition (with Model Oil 2) in a Cold Finger In order to explain the different characteristics of the water content of the deposit observed in this investigation (water content of the deposit equals that of the bulk) with the observations in literature (water content of the deposit does not correlate with that of the bulk), we hypothesize that the water content of the deposit depends on the droplet size of the bulk. Intuitively, droplets smaller than the void space of the deposit solid matrix can be entrapped by deposition. Wax deposition experiments with various droplet size were then performed to test this hypothesis.


Energy & Fuels

11-18-2016 The size of the water droplets in the bulk was varied by varying the surfactant loading within a range of 0.1 wt.% to 1.0 wt.%. It should be noted that with low surfactant dosages (< 0.5wt.%), the resulting water-in-oil dispersions undergo rapid phase separation when kept stationary. Therefore, model oil 2 with a WAT above the room temperature was used for this investigation of the effect of droplet size on the entrapment of water during wax deposition. Because the WAT of model oil 2 is above the room temperature, wax particles precipitate and act as barriers against flocculation and coalescence of water droplets, retarding phase separation and allowing for droplet size characterization. Figure 12 shows the variation of water content of the deposit as a function of surfactant loading. Recall that the water-in-oil mixtures prepared with surfactant concentrations at 0.5 wt.% and 1 wt.% are termed as “water-in-oil emulsions” as they present long-term stability against phase separation. The water-in-oil mixtures prepared with surfactant concentrations below 0.5 wt.% are termed as “water-in-oil dispersions” as they undergo rapid phase separation when kept stationary.

60 Water Content (wt.%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 59

50 54 wt.%, calculated based on the mass of water and oil added to the cold finger

40

Bulk

30 Bulk

20

Deposit

Deposit

10 0 0

0.2

0.4

0.6

0.8

1

Amount of Surfactant (wt.%)

Figure 12: Effect of Surfactant Concentration on the Water Content of the Deposit Generated from an Emulsion or Dispersion with Water Content of 50 vol.% (54 wt.%) 20 ACS Paragon Plus Environment

Page 21 of 59

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

11-18-2016 As can be seen from Figure 12, regardless of the surfactant concentration, the water content of the bulk always equals the water content calculated based on the amount of water and oil added to the cold finger. Based on this observation, it can be concluded that although surfactant concentrations below or equal to 0.2 wt.% are not sufficient to generate stable emulsions, mixing provided by the stir plate can maintain homogeneous dispersions during wax deposition experiments. It can also be observed from Figure 12 that the water content of the deposit increases with increasing surfactant concentration. When the surfactant concentration increases to as high as 0.5 wt.%, the water content of the deposit equals that of the emulsion as measured by Karl-Fisher titration. The droplet size of the dispersion decreases with increasing surfactant concentration as the surfactant lowers the interfacial tension between oil and water, making it easier for fine droplets to form. Therefore, it may be deduced that the water content of the deposit increases with decreasing droplet size in the bulk, i.e., fine droplets are easier to be incorporated in the deposit than coarse droplets. If this hypothesis holds, the droplet size distribution of the deposit should be smaller than that of the dispersion. In order to test this expectation, we proceeded to measure the droplet size distributions of the deposit and the emulsion/dispersion. The droplet size distribution has to be measured with the DOSY-NMR technique as the conventional microscopy experiments are not applicable owing to the following reason. Measuring droplet size distribution with conventional microscopic experiments requires melting of suspended wax particles. Unfortunately, with the low surfactant dosages (0.1 wt.% - 0.2 wt.%) in the oil phase, phase separation occurs rapidly upon melting of wax particles.


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

11-18-2016 DOSY-NMR does not require the wax particles to be melted for droplet size distribution characterizations. Solid wax particles can then act as barriers to flocculation/coalesce of water droplets and retards phase separation, shown in Figure 13.

Figure 13: Bulk Appearance of Water-in-Oil Dispersion Prepared with a Low Surfactant Dosage at 0.1 wt.% after Kept Stationary for 30 Minutes: (a) Bulk Stability of Water-in-Oil Dispersion in Presence of Suspended Wax Particles and (b) Rapid Phase Separation of Oil and Water in Absence of Wax Particles DOSY-NMR measures the droplet size based on 1H signal attenuation under pulse field gradient due to the Brownian motion of water molecules in the droplet. Brownian motions of the water molecules enclosed by fine droplets are expected to generate a smaller signal attenuation than that generated by water molecules enclosed by coarse droplets. In order to demonstrate the effect of droplet diameter on the 1H signal attenuation, DOSY-NMR measurements were taken for a stable emulsion prepared with a surfactant concentration of 0.5wt.%, a dispersion with a surfactant concentration of 0.2wt.% and DI water. Figure 14 shows the comparison between the acquired 1H2O proton NMR signal attenuations from these three samples.


Page 22 of 59

Page 23 of 59

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

11-18-2016

Figure 14: An Illustration of the Effect of Droplet Diameter on the 1H Signal Attenuation of the H2O Molecules Enclosed by Droplets As can be seen from Figure 14, the minimal signal attenuation was observed with the emulsion sample prepared with a surfactant concentration of 0.5 wt.%, indicating a finer droplet diameter than the dispersion prepared with a surfactant concentration of 0.2 wt.%. The DI water sample generates the largest signal attenuation as the Brownian motions of the water molecules are not restricted by any boundary at all. The measured NMR signal attenuation can then be used to back-calculate the droplet size distribution. Figure 15 shows the measured signal attenuation of the water molecules in the emulsion/dispersion and deposit formed with four different surfactant concentration: 0.5wt.%, 0.2wt.%, 0.15wt.% and 0.1 wt.%.


Energy & Fuels

11-18-2016 1

Signal Attenuation (I/I0)


1

0.8

0.6

0.5wt.% Surfactant

0.4

Emulsion

Deposit

0.2

0.8

0.6

0.2wt.% Surfactant

0.4

Dispersion

Deposit

0.2 0

1

2 4 6 Pulse Field Gradient (Gauss/cm)

8

0


1

(a)

8

(b)

1


1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 59

0.8

0.6

0.15wt.% Surfactant

0.4

Dispersion

Deposit

0.2

0.8

0.6

0.1wt.% Surfactant

0.4

Dispersion

Deposit

0.2 0


8

0


(c)

8

(d)

Figure 15: Comparisons between the 1H Signal Attenuations of H2O Molecules in the Emulsion/Dispersion and Deposits under Pulse Field Gradients As can be seen from Figure 15 (a), the 1H signal attenuations of the water molecules in the emulsion and in the deposit generated with a surfactant concentration of 0.5wt.% are virtually the same, indicating identical droplet size in the emulsion and the deposit. The signals obtained from the deposit samples always decrease at a slower rate with increasing pulse field gradient when the surfactant concentration is less than or equal to 0.2 wt.%, as seen in Figure 15 (b)-(d), qualitatively suggesting that the deposit droplet size is smaller than the bulk droplet size. Figure 16 shows the quantitative droplet size distribution of the emulsion/dispersion and deposits back-calculated from the measured signal attenuations.


Page 25 of 59

11-18-2016 Surfactant Concentration = 0.5wt.%

Surfactant Concentration = 0.2wt.% 0.06

Emulsion Deposit

0.15

Probability Density

Probability Density

0.2

0.1 0.05 0 0

0.04 0.02 0 0

100 200 300 Droplet Diameter (µ m) Surfactant Concentration = 0.1wt.%

400

0.06 Probability Density

Dispersion Deposit 0.02 0.01 0 0

Dispersion Deposit

100 200 300 400 Droplet Diameter (µ m) Surfactant Concentration = 0.15wt.%

0.03 Probability Density

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

100 200 300 Droplet Diameter (µ m)

Dispersion Deposit 0.04 0.02 0 0

400

100 200 300 Droplet Diameter (µ m)

400

Figure 16: Comparison between the Droplet Size Distributions of the Emulsion/Dispersion and the Deposit Generated from the Corresponding Emulsion/Dispersion As can be seen from Figure 16, the droplet size distribution of the deposit is smaller and narrower than or equal to that of the dispersion in all experimental runs, supporting our initial hypothesis that fine droplets are more easily incorporated in the deposit than coarse droplets. Moreover, the size of the droplets that can be incorporated in the deposit is usually smaller than ~100 microns. Recall that no obvious correlation between the water content of the bulk and that of the deposit was observed in previous studies while we have observed same water contents of the bulk and deposit. This inconsistency can now be explained: at low surfactant dosages, coarse droplets are generated but cannot be trapped by deposit formation, leading to a lower water content of the deposit than that of the oil. It should be noted that the droplet size of a dispersion formed with a particular crude oil depends on the intrinsic emulsifying power of the natural surfactants contained by this crude oil.


Energy & Fuels

11-18-2016 4. ROLE OF WATER DROPLETS ON THE SHEARING AND SLOUGHING OF WAXY-EMULSION GELS Entrapment of water droplets in the deposit significantly lowers its yield stress, as is denoted by the blue circles in Figure 17. It was initially suspected that the lowered yield stress of the waxy emulsion gel is due to the addition of surfactants. However, follow-up experiments indicated that the addition of surfactant to a concentration of up to 1 wt.% has negligible impact on the yield stress of the gel, as is denoted by the red triangle in Figure 17.

Yield Stress (Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Red triangle: Oil with surfactant

Figure 17: Effect of Water Content of the Waxy Emulsion Gel on the Yield Stress of the Gel As can be seen from Figure 17, the yield stress of the waxy emulsion gel decreases with increasing water content of the gel. The waxy emulsion gel at a water content of 70 vol.% has a yield stress (30 Pa) that is less than 10% that of the waxy gel free of water (510 Pa). These two gels also have distinct appearances after breakage, shown in Figure 18.


Page 26 of 59

Page 27 of 59

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

11-18-2016

Figure 18: Appearances of the Water Free Waxy Gel and Waxy Emulsion Gel after Gel Breakage As can be seem from Figure 18 (a), the water free waxy gel appears to be mostly solid-like with debris broken from the edge of the gel. On the contrary, the emulsion gel with a water content of 70 vol.% becomes slurry-like after gel breakage, as is seen in Figure 18 (b). A lowered yield stress can cause the deposit to become prone to be “sloughed off” by the shear stress imposed by the fluid flow, leading to a significantly lower wax deposit build-up predicted by conventional wax deposition models. The “sloughing-off” of wax deposit containing water droplets in the deposit was also investigated with a flow loop apparatus in the section 5 below.

5. FLOW LOOP WAX DEPOSITION EXPERIMENTS Wax deposition experiments were performed with water-in-oil dispersion (prepared at a surfactant concentration of 0.1wt.%). Figure 19 shows the pressure-time trajectory for wax deposition experiments carried out with water content of the bulk at 10 vol.%. In the experimental runs performed with and without water in the bulk, the Reynolds number


Energy & Fuels

11-18-2016 associated with the pipe flow is approximately 300. In other words, wax deposition experiments were performed in laminar flow regime. 20

Pressure Drop (psi)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

16

deposition runs without water

12 8

deposition runs with 10 vol.% water dispersed in oil

4 0 0

0.2

0.4 0.6 Time (hour)

0.8

1

Figure 19: Pressure-Time Trajectories for Wax Deposition Experiments with Various Water Contents of the Bulk at the Following Operating Conditions: Toil = WAT+5°C, Temperature of the Coolant Stream, Tcoolant = WAT-30°C and Oil Flow Rate Qoil = 0.2 m3/hr As can be seen from the experiments with water-in-oil dispersion, the pressure drop increases due to deposit build-up and then decreases abruptly multiple times during the experiment. These abrupt drops in the pressure transducer reading serve as the first evidence of deposit slough-off. In order to confirm this slough-off phenomenon, we also meticulously inspected the times at which these rapid drops in pressure transducer occurred and looked for other evidence that suggest “slough-off”. It was discovered that the thermal couple readings at the test section outlet decreased immediately after each and every decrease in the pressure transducer reading, shown in Figure 20.


Page 28 of 59

Page 29 of 59

Pressure Drop (psia)

11-18-2016

Temperature (o C)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Energy & Fuels

Figure 20: Correlation between the Abrupt Decreases in the Pressure Transfer and Outlet Thermal Couple Readings Supports the Hypothesis that Deposit Sloughed off Under Imposed Shear


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

11-18-2016 This correlation between the drops in pressure and temperature readings indicates that a small piece of deposit was first sloughed off from the cold pipe wall/deposit-fluid interface, leading to less restriction of the pipe flow and a corresponding decrease in the pressure drop. This debris then traveled with the fluid flow and eventually reached the outlet thermal couple. The contact of the wax deposit debris with the thermal couple causes a decrease in the thermal couple reading because this debris is removed from the cold wall and is at a lower temperature than the bulk. It should be noted that no such correlated drops in the pressure transducer and thermal couple readings were observed for single phase wax deposition experiments, shown in Figure 21.


Page 30 of 59

Page 31 of 59

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Energy & Fuels

11-18-2016

No decrease in pressure

No decrease in temperature

Figure 21: Pressure Drop and Thermal Couple Readings during a Single Phase Wax Deposition Experiments with the Following Operating Conditions: Toil = WAT+5°C, temperature of the coolant stream, Tcoolant = WAT-30°C and oil flow rate Qoil = 0.2 m3/hr


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

11-18-2016 The simultaneous drops in the pressure and the outlet temperature confirm slough-off. It should be noted that characterization of the deposit shows a water content of as low as 5 wt.%, suggesting a small amount of water can drastically lower the yield stress of the deposit and make it prone to slough-off.

6. CONCLUSIONS In this investigation, we presented the most comprehensive characterization of the entrapment of water droplets in the wax deposit during wax deposition from water-in-oil emulsion/dispersion. This characterization resolves inconsistencies in the water content of the deposit measured in literature. The key findings in this investigation are: 1) The upper limit of the water content of the deposit equals the water content of the bulk. 2) When the water droplets of the dispersion are coarse, the deposit will contain virtually no water even with a bulk water content of as high as 50 vol.%. 3) Comparison between the bulk and deposit droplet size distributions suggests that only the droplets smaller than 100 microns in diameter can be incorporated in the deposit. In addition, based on the rheological characterizations as well as flow loop experiments, the following roles of the dispersed water phase on wax deposit build up are reported. 4) Incorporation of the water droplets in the deposit leads to a significant decrease in the deposit yield stress. 5) This decrease in the deposit yield stress causes deposit to become prone to be sloughed off during deposit build up, leading to a lower overall deposition rate. Figure 22 highlights the key points addressed in this investigation.


Page 32 of 59

Page 33 of 59

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

11-18-2016 Oil

slough-off deposition

Key point 1: How much water in the deposit?

breakage

Key point 3: How does incorporation of water droplets affect the sloughoff of deposit?

Key point 2: How does the deposit yield stress change with the water content?

Figure 22: Highlights of the Key Points Addressed in this Investigation: 1) Incorporation of the Water Droplets in the Deposit, 2) Lowered Yield Stress of Deposit due to Incorporation of Water Droplets and 3) Slough-off of Deposit due to the Lowered Yield Stress We may expect that with a crude oil rich in natural surfactants, the wax deposit formed from water-in-crude oil dispersion/emulsion can contain a large number of fine droplets and become prone to slough-off. Without the consideration for deposit slough-off, existing wax deposition models can significantly over-predict the deposit growth rate, resulting in conservative design. This over-prediction is likely to occur when the crude oil of interest contains a large amount of natural surfactants. The deposits formed with such crude oils are then expected to entrap a large amount of water droplets, leading to frequent sloughing. It should be noted that the slough-off of wax deposit is random. Modeling of such stochastic process is beyond the scope of the current investigation. Such modeling campaign likely requires advanced techniques in probability theory.

7. ACKNOWLEDGEMENT SZ, TK and HSF would like to thank Dr. Mark R. Jemmett for his efforts in designing, building and testing the flow loop apparatus. SZ, TK and HSF would like to thank the financial and intellectual supports from the University of Michigan Industrial Affiliate 33 ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

11-18-2016 Program Members: AssuredFlowSolutions LLC., BP, Chevron, ConocoPhillips, Multichem a Hallibruton Service, Phillips66, Statoil and Total. SZ would like to thank Dan Coy from BP and Steve Appleyard from ConocoPhillips for the helpful discussions on the safety aspects of the flow loop apparatus, and Lisa Stowe from OSEH at University of Michigan for performing the safety inspection and certifying the flow loop apparatus. SZ would like to thank Dr. Gesine Veits and Professor Anne McNeil for their supports on the rheological tests. SZ would also like to thank Dr. Christopher Kojiro and Dr. Eugneio Alvarado at the University of Michigan NMR facility for their help on the DOSY-NMR experiments.


Page 34 of 59

Page 35 of 59

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

11-18-2016 8. REFERENCES 1

Mohamed, A. M. O.; Elgamal, M.; Said, R. . Determination of water content and salinity from a producing oil well using CPW probe and eigendecomposition Sensors Actuators A 2006, 125, 133–142.

2

Bailey B.; Crabtree M.; Tyrie J.; Kuchuk, F.; Romano, C.; Roodhart, L. Water Control, Oilfield Review 12 (Spring 2000) 30-51.

3

Craig Jr., F. F. 1971. The Reservoir Engineering Aspects of Waterflooding, Vol.3. Richardson, Texas: Monograph Series, SPE

4

Whillhite, G. P. 1986. Waterflooding, Vol.3. Richardson, Texas: Textbook Series, SPE

5

Rose, S. C., Buckwalter, J. F., and Woodhall, R. J. 1989. The Design Engineering Aspects of Waterflooding, Vol. 11. Richardson, Texas: Monograph Series, SPE

6

“Controlling excess water production” Petrowiki, SPE, http://petrowiki.org/Controlling_excess_water_production#cite_note-r1-1, Accessed on April-20-2016

7

Panacharoensawad, E.; Sarica, C. Experimental Study of Single-Phase and Two-Phase Waterin-Crude- Oil Dispersed Flow Wax Deposition in a Mini Pilot-Scale Flow Loop Energy & Fuels 2013, 27, 5036–5053..

8

Wang, P.; Wang, W.; Gong, J.; Zhou, Y.; Yang, W. Effects of the Dispersed Phase on Oil/Water Wax Deposition J. Energy Resour. Technol. 2013, 135, 042902.

9

Abdurahman, N. H.; Rosli, Y. M.; Azhari, N. H.; Hayder, B. a. Pipeline transportation of viscous crudes as concentrated oil-in-water emulsions J. Pet. Sci. Eng. 2012, 90-91, 139– 144.

10

Vilasau, J.; Solans, C.; Gómez, M. J.; Dabrio, J.; Mújika-Garai, R.; Esquena, J. Stability of oilin-water paraffin emulsions prepared in a mixed ionic/nonionic surfactant system Colloids Surfaces A Physicochem. Eng. Asp. 2011, 389, 222–229.

11

Ghosh, S.; Mandal, T. K.; Das, G.; Das, P. K. Review of oil water core annular flow Renew. Sustain. Energy Rev. 2009, 13, 1957–1965.

12

Hoffmann, R.; Amundsen, Lene; Huang, Z.; Zheng, S.; Fogler, H. S. Wax Deposition in Stratified Oil/Water Flow Energy & Fuels 2012, 26, 3416–3423.

13

Czarnecki, J.; Tchoukov, P.; Dabros, T.; Xu, Z. Role of asphaltenes in stabilisation of water in crude oil emulsions Can. J. Chem. Eng. 2013, 91, 1365–1371.

14

McLean, J. D.; Kilpatrick, P. K. Effects of Asphaltene Solvency on Stability of Water-inCrude-Oil Emulsions J. Colloid Interface Sci. 1997, 189, 242–253.

15

Schorling, P.-C.; Kessel, D. .; Rahimian, I. Influence of the crude oil resin/asphaltene ratio on the stability of oil/water emulsions Colloids Surfaces A Physicochem. Eng. Asp. 1999, 152, 95–102.

16

Xia, L.; Lu, S.; Cao, G. Stability and demulsification of emulsions stabilized by asphaltenes or resins. J. Colloid Interface Sci. 2004, 271, 504–506.


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

11-18-2016 17

Bruno, A.; Sarica, C.; Chen, H.; Volk, M. Paraffin Deposition During the Flow of Water-inOil and Oil-in-Water Dispersions in Pipes SPE114747 2008, .

18

Couto, G. H.; Chen, H.; Dellecase, E.; Sarica, C.; Volk, M. An Investigation of Two-Phase Oil / Water Paraffin Deposition. In 2006 Offshore Technology Conference: Houston, Texas, USA; Offshore Technology Conference: 2008.

19

Panacharoensawad, E.; Sarica, C. Experimental Study of Single-Phase and Two-Phase Waterin-Crude- Oil Dispersed Flow Wax Deposition in a Mini Pilot-Scale Flow Loop Energy & Fuels 2013, 27, 5036–5053.

20

Zhang, Y.; Gong, J.; Ren, Y.; Wang, P. Effect of Emulsion Characteristics on Wax Deposition from Water-in-Waxy Crude Oil Emulsions under Static Cooling Conditions Energy & Fuels 2010, 24, 1146–1155.

21

Kasumu, A. S.; Mehrotra, A. K. Solids Deposition from Two-Phase Wax–Solvent–Water “Waxy” Mixtures under Turbulent Flow Energy & Fuels 2013, 27, 1914–1925.

22

Wang P, Wang W, Gong J, Zhou Y, Yang W. Effects of the Dispersed Phase on Oil/Water Wax Deposition. J Energy Resour Technol. 2013;135(4):42902. doi:10.1115/1.4023932.

23

Quan Q, Wang W, Wang P, Zhou Y, Gong J. Wax Deposition of Water-in-crude Oil Emulsion in a Flow-loop Apparatus. Pet Sci Technol. 2015;33(5):520-526. doi:10.1080/10916466.2014.999944.

24

Wang Z, Liu Y, Li J, Zhuge X, Zhang L. Study on Two-Phase Oil–Water Gelling Deposition Behavior in Low-Temperature Transportation. Energy & Fuels. 2016;30(6):4570-4582. doi:10.1021/acs.energyfuels.6b00294.

25

Oh, K.; Deo, M. D. Yield behavior of gelled waxy oil in water-in-oil emulsion at temperatures below ice formation Fuel 2011, 90, 2113–2117.

26

Sun, G.; Zhang, J.; Li, H. Structural behaviors of waxy crude oil emulsion gels Energy and Fuels 2014, 28, 3718–3729.

27

Venkatesan, R. The Deposition and Rheology of Organic Gels. University of Michigan, 2004.

28

Paso, K.; Silset, A.; Sørland, G.; Goncalves, M. de A. L.; Sjöblom, J. Characterization of the Formation , Flowability , and Resolution of Brazilian Crude Oil Emulsions 2009, 471–480.

29

Delgado-Linares, J. G. J. G.; Majid, A. a. a.; Sloan, E. D.; Koh, C. a.; Sum, A. K. Model Water-in-Oil Emulsions for Gas Hydrate Studies in Oil Continuous Systems Energy & Fuels 2013, 27, 4564–4573.

30

Zheng, S.; Fogler, H. S. Fundamental Investigation of Wax Diffusion Characteristics in Waterin-Oil Emulsion Ind. Eng. Chem. Res. 2014, ASAP, .

31

Packer, K.; Rees, C. Pulsed NMR Studies of Restricted Diffusion. I. Droplet Size Distributions in Emulsions J. Colloid Interface Sci. 1972, 40, 206–218.

32

Opedal, N. V. der T.; Sørland, G.; Sjöblom, J. Methods for Droplet Size Distribution Determination of Water-in-oil Emulsions using Low-Field NMR Open-Access J. Basic Princ. Diffus. Theory, Exp. Appl. 2009, 9, 1–29.

33

Aichele, C. P.; Chapman, W. G.; Rhyne, L. D.; Subramani, H. J.; House, W. V. Analysis of Formation of Water-in-Oil Emulsions Energy & Fuels 2009, 23, 3674–3680. 36 ACS Paragon Plus Environment

Page 36 of 59

Page 37 of 59

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

11-18-2016 34

Singh, P.; Venkatesan, R.; Fogler, H. S.; Nagarajan, N. Formation and aging of incipient thin film wax-oil gels AIChE J. 2000, 46, 1059–1074.

35

M. Lashkarbolooki, A. Seyfaee, F. Esmaeilzadeh, and D. Mowla, Experimental Investigation of Wax Deposition in Kermanshah Crude Oil through a Monitored Flow Loop Apparatus, Energy Fuels 2010, 24, 1234–1241.

36

Creek, J. L.; Lund, H. J.; Brill, J. P.; Volk, M. Wax deposition in single phase flow, Fluid Phase Equilib.1999, 158-160, 801.

37

M. Lashkarbolooki , F. Esmaeilzadeh and D. Mowla, Mitigation of Wax Deposition by WaxCrystal Modifier for Kermanshah Crude Oil, Journal of Dispersion Science and Technology, 32:975–985, 2011.


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1: Summary of Reported Characterizations of the Water Content in the Deposit in Comparison with the Water Content of the Dispersion Figure 1 159x129mm (96 x 96 DPI)

ACS Paragon Plus Environment

Page 38 of 59

Page 39 of 59

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure 2: Carbon Number Distributions of the Waxes Used in the Preparation of Model Oils Figure 2 161x126mm (150 x 150 DPI)


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3: A schematic Drawing of the Cold Finger Apparatus Figure 3 170x147mm (96 x 96 DPI)


Page 40 of 59

Page 41 of 59

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure 4: Schematic Drawing of the Michigan Flow Loop. Labeled Sections Refer to: 1) Conditioning System, 2) Pumping System, 3) Testing System and 4) Data Acquisition System Figure 4 244x141mm (92 x 92 DPI)


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5: Determination of the Yield Stress of a Waxy Gel/Waxy Emulsion Gel Based on the Viscosity-Stress Trajectory Figure 5 179x134mm (150 x 150 DPI)


Page 42 of 59

Page 43 of 59

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure 6: Comparison between the Water Content in the Deposit with that in the Emulsion Figure 6 223x131mm (150 x 150 DPI)


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 7: Comparison between the Microstructures of Deposit and Emulsion Figure 7 273x90mm (150 x 150 DPI)


Page 44 of 59

Page 45 of 59

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure 8: Comparison between the Droplet Size Distributions of the Deposit and the Emulsion Figure 8 214x181mm (150 x 150 DPI)


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 9: Time Evolution of the Deposit Weight and Wax Content with a Water Content of 10 vol.% Figure 9 177x128mm (150 x 150 DPI)


Page 46 of 59

Page 47 of 59

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure 10: Time Evolutions of (a) Water Content and (b) Droplet Size Distribution of the Deposit Generated from Emulsion with Water Content of 10 vol.% Figure 10 291x129mm (150 x 150 DPI)


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 11: Time Evolution of Water Content of the Deposit Generated from Emulsion with Water Contents of 10, 30, 50 and 70 vol.% Figure 11 226x109mm (150 x 150 DPI)


Page 48 of 59

Page 49 of 59

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure 12: Effect of Surfactant Concentration on the Water Content of the Deposit Generated from an Emulsion or Dispersion with Water Content of 50 vol.% (54 wt.%) Figure 12 213x132mm (150 x 150 DPI)


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 13: Bulk Appearance of Water-in-Oil Dispersion Prepared with a Low Surfactant Dosage at 0.1 wt.% after Kept Stationary for 30 Minutes: (a) Bulk Stability of Water-in-Oil Dispersion in Presence of Suspended Wax Particles and (b) Rapid Phase Separation of Oil and Water in Absence of Wax Particles Figure 13 235x134mm (150 x 150 DPI)


Page 50 of 59

Page 51 of 59

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure 14: An Illustration of the Effect of Droplet Diameter on the 1H Signal Attenuation of the H2O Molecules Enclosed by Droplets Figure 14 287x134mm (150 x 150 DPI)


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 15: Comparisons between the 1H Signal Attenuations of H2O Molecules in the Emulsion/Dispersion and Deposits under Pulse Field Gradients Figure 15 257x191mm (150 x 150 DPI)


Page 52 of 59

Page 53 of 59

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure 16: Comparison between the Droplet Size Distributions of the Emulsion/Dispersion and the Deposit Generated from the Corresponding Emulsion/Dispersion Figure 16 316x155mm (128 x 129 DPI)


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 17: Effect of Water Content of the Waxy Emulsion Gel on the Yield Stress of the Gel Figure 17 158x118mm (150 x 150 DPI)


Page 54 of 59

Page 55 of 59

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure 18: Appearances of the Water Free Waxy Gel and Waxy Emulsion Gel after Gel Breakage Figure 18 233x132mm (150 x 150 DPI)


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 19: Pressure-Time Trajectories for Wax Deposition Experiments with Various Water Contents of the Bulk at the Following Operating Conditions: Toil = WAT+5°C, Temperature of the Coolant Stream, Tcoolant = WAT-30°C and Oil Flow Rate Qoil = 0.2 m3/hr Figure 19 294x110mm (150 x 150 DPI)


Page 56 of 59

Page 57 of 59

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure 20: Correlation between the Abrupt Decreases in the Pressure Transfer and Outlet Thermal Couple Readings Supports the Hypothesis that Deposit Sloughed off Under Imposed Shear Figure 20 307x185mm (150 x 150 DPI)


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 21: Pressure Drop and Thermal Couple Readings during a Single Phase Wax Deposition Experiments with the Following Operating Conditions: Toil = WAT+5°C, temperature of the coolant stream, Tcoolant = WAT-30°C and oil flow rate Qoil = 0.2 m3/hr Figure 21 313x173mm (150 x 150 DPI)


Page 58 of 59

Page 59 of 59

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure 22: Highlights of the Key Points Addressed in this Investigation: 1) Incorporation of the Water Droplets in the Deposit, 2) Lowered Yield Stress of Deposit due to Incorporation of Water Droplets and 3) Slough-off of Deposit due to the Lowered Yield Stress Figure 22 322x115mm (96 x 96 DPI)


Entrapment of Water Droplets in Wax Deposits ... - ACS Publications

Recommend Documents